Astrophysicists explain the differences in the brightness of supernova explosions

Astrophysicists explain the differences in the brightness of supernova explosions

Supernovae
stand out in the sky like cosmic lighthouses. Scientists at the Max
Planck Institute for Astrophysics and at the National Astronomical
Institute of Italy have now found a way to use these cosmic beacons to
measure distances in space more accurately. The researchers have been
able to show that all supernovae of a certain type explode with the
same mass and the same energy - the brightness depends only on how much
nickel the supernova contains. This knowledge has allowed the
researchers to calibrate the brightness of supernovae with greater
precision. This means that in the future, they will use the brightness
of a supernova that they are observing through their telescopes to
determine more accurately how far away from the Earth the cosmic
lighthouse is emitting its rays (Science, 9 February 2007).

Fig:
The
arrow points to the supernova 2002bo, the explosion of a white dwarf in
the galaxy NGC 3190 in the Leo constellation - 60 million light years
away from earth.
Image: Benetti et al., MNRAS 384, 261-278 (2004)

The end of a star's life, when the star has become heavy enough, is
marked by a huge explosion - a supernova. For a few weeks, a supernova
looks almost as bright as a whole galaxy containing billions of stars.
Physicists designate the brightest of these supernovae as Type Ia.
Their brightness, measured from the Earth, is a measure of their
distance from us - but there are several uncertainties. "The question
still remains: how suitable are supernovae really for measuring
distance? For example, the knowledge that the Universe is expanding
rapidly is largely based on observations of supernovae," explains Prof.
Wolfgang Hillebrandt. All type Ia supernovae exhibit similar levels of
brightness, but they are not exactly consistent.

Scientists
from the Max Planck Institute for Astrophysics and the National
Astronomical Institute of Italy have now made a breakthrough. They have
come to the conclusion that the explosion energy of the type Ia
supernovae is almost consistent - it is equivalent to the fusion energy
which a white dwarf with around one and half times the mass of the Sun
can develop. However, the amount of radioactive nickel and
medium-weight chemical elements such as silicon vary from supernova to
supernova and explain the difference in their brightness. The more
nickel a supernova contains, the brighter it shines.

In the
explosion, nuclear fusion of carbon and oxygen creates large quantities
of radioactive atomic nuclei; in some supernovae, this is mainly the
radioactive isotope 56 of the element nickel. The energy from its
radioactive decay is converted to light in the supernova. The fusion
therefore supplies both the energy and the light for the explosion. The
nuclear fusion, however, can end with lighter atomic nuclei like
silicon, for example. This creates the same amount of energy, but the
supernova is not so bright. The researchers identify this situation
when they also see the silicon in the light spectrum of the supernova.

Over
the last four years, in a study forming part of a European joint
venture lead by the Max Planck Institute for Astrophysics, scientists
have looked at 20 Ia supernova explosions, following each one for
several weeks. Using spectroscopic and photometric data and complicated
numerical simulations, they arrived at results that now make it
possible to refine existing calibration methods. Astronomers calibrate
the differences in brightness of the supernovae with their light
curves; that is, the way the brightness develops over time in newly
discovered supernovae. The light curves of brighter supernovae diminish
more slowly than those of less bright supernovae. Up to now, the
weakest link in this calibration method has been limited knowledge
about the supernova explosions themselves: what causes the differences
in brightness and are the corrections made to them justified? The
supernovae that play a part in cosmology in measuring distances
exploded just as our solar system was coming into existence, or even
earlier. Consequently, there is no guarantee that these are the same
explosions as those for which the light curves have been calibrated.

In
order to exclude possible systematic differences, scientists need to
have a very good understanding of the explosions, and the scientists
from the Max Planck Institute for Astrophysics and the National
Astronomical Institute of Italy have now made a large contribution to
this. "Our surprising results have for the first time delivered a solid
basis on which we can use supernovae to measure distances in space,"
says Wolfgang Hillebrandt. "We now understand the differences in the
brightness of supernovae better and can calibrate this cosmic yardstick
accurately in the future." These findings will also benefit
cosmologists who use the brightness of supernovae to deduce dark
energy. Scientists believe that it is this dark matter that is
responsible for the rapid expansion of the Universe.